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Figure 6. Relative contribution of the metacarpal (black), proximal phalanx (shaded), and middle phalanx (white) in the hand of didelphids and cheirogaleids. The values for rays II through V represent mean percentages of total ray length for each species.

mammals that inhabit fine-branch niches (i.e., Marmosa and Caluromys). This suggests to us that the invasion of a fine-branch niche by the earliest primates led to the evolution of more prehensile hands and feet (i.e., longer digits relative to the metapodials)—a conclusion also reached by Hamrick (2001). In the following section, we present behavioral observations and tests using the same contrasts among didelphids in order to strengthen the functional link between having more prehensile hands and feet (i.e., longer digits relative to the palm/sole) and improved abilities to negotiate thin branches.